1. The Field of the Invention
The present invention relates to vertical diodes and more specifically to vertical diodes with low series resistance formed on a silicon wafer.
2. The Relevant Technology
One of the common trends in the electronics industry is the miniaturization of electronic devices. This trend is especially true for electronic devices operated through the use of semiconductor microchips. Microchips are commonly viewed as the brains of most electronic devices. In general, a microchip comprises a small silicon wafer upon which can be built thousands of microscopic electronic devices that are integrally configured to form electronic circuits. The circuits are interconnected in a unique way to perform a desired function.
With the desire to decrease the size of electronic devices, it is also necessary to decrease the size of the microchip and electronic devices thereon. This movement has increased the number and complexity of circuits on a single microchip.
One common type of electronic device found on a microchip is a diode. A diode functions as a type of electrical gate or switch. An ideal diode will allow an electrical current to flow through the diode in one direction but will not allow an electrical current to flow through the diode in the opposite direction. In conventional diodes, however, a small amount of current flows in the opposite direction. This is referred to as current leakage.
Conventional diodes are typically formed from a silicon material that is modified through a doping process. Doping is a process in which ions are implanted within the silicon. There are two general types of dopants: P-type dopants and N-type dopants. P-type dopants are materials that when implanted within the silicon produce regions referred to as holes. These holes can freely accept electrons. In contrast, N-type dopants are materials that when implanted within silicon produce extra electrons. The extra electrons are not tightly bound and thus can easily travel through the silicon. In general, a diode is formed when a material doped with a P-type dopant is connected to a material doped with an N-type dopant.
Conventional diodes are configured by positioning the two opposing doped materials side by side on a microchip. This side by side positioning, however, uses a relatively large amount of surface space on the microchip. As a result, larger microchips are required.
Furthermore, for a diode to operate, each side of the diode must have an electrical connection that either brings electricity to or from the diode. The minimal size of each side of the diode is in part limited in that each side must be large enough to accommodate an electrical connection. Since conventional diodes have a side by side configuration with each side requiring a separate electrical connection, the ability to miniaturize such diodes is limited. In addition, the requirement of having side by side electrical connections on a single diode increases the size and complexity of the microchip.
Attempts have been made to increase the efficiency and current flow rate through a diode so as to speed up the microchip. In one attempt to accomplish this end, one of the sides of the diode is heavily doped and the other side of the diode is lightly doped. The lightly doped side limited the current, and the heavily doped side increased the reverse bias leakage. Thus, such a configuration produces minimal gain.
Other attempts have been made to decrease the resistance in the above discussed diode by increasing the dopant concentration on the lightly doped side of the diode. As the dopant concentration is increased, however, current leakage in the diode increases. In turn, the current leakage decreases the current efficiency and functioning of the microchip.
It is therefore an object of the present invention to provide improved diodes and their method of manufacture.
Another object of the present invention is to provide improved diodes that use a minimal amount of surface area on a microchip.
Still another object of the present invention is to provide improved diodes that are easily connected to other electronic devices of an integrated circuit.
Also another object of the present invention is to provide improved diodes having improved current flow and efficiency.
It is another object of the present invention to provide improved diodes having a heavily doped area and a lightly doped area with minimal resistance and current leakage.
Yet another object of the present invention is to provide improved diodes that can be selectively sized.
Finally, another object of the present invention is to provide improved diodes having a minimal cost.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.
In order to achieve the above objectives and in accordance with the invention as claimed and broadly described herein, a vertical diode is provided on a silicon wafer. The silicon wafer is doped with a first type of dopant and has an exposed surface. A vertical diode incorporating features of the present invention is manufactured by initially highly doping the exposed surface of the silicon wafer with a second type of dopant to form an active region.
Next, the active region is covered by a refractory metal silicide layer, preferably titanium silicide. The silicide layer has a relatively low resistance and, thus, ultimately decreased the resistance through the vertical diode. An insulation layer, such as silicon dioxide, is then formed over the refractory metal silicide layer. The insulation layer is formed using conventional oxidation deposition processes. A conventional masking and etching process is used to etch a diode trench through the insulation layer so as to expose a portion of the refractory metal silicide layer. The diode trench is defined by an interior surface which contacts the refractory metal silicide layer.
The diode trench is next filled with amorphous silicon which is then lightly doped with the second type of dopant. The amorphous silicon forms a silicon plug within the diode trench. The silicon plug has a bottom portion contacting the refractory metal silicide layer and a top portion separated from the refractory metal silicide layer by the bottom portion.
The amorphous silicon is next heated to recrystallize the amorphous silicon into large grain polysilicon. The second portion of the silicon plug, now converted into polysilicon, is then heavily doped with the first type of dopant. The doping is performed by ion implantation followed by a heat treatment, such as RTP, for activation of the dopant. Finally, a metal contact is secured to the top portion of the silicon plug to complete the vertical diode.
Since the diode has a vertical formation, use of the surface area on the silicon microchip is minimized. Furthermore, as there is only one connection point on top of the diode, the diode is easier to connect to other elements and is easier to size.
In one alternative embodiment, a programmable resistor is positioned between the metal contact and the top portion of the silicon plug. The programmable resistor comprises chalcogenide material and barrier materials. One preferred barrier material is titanium nitride. The programmable resistor allows the diode to have memory characteristics.
In yet another alternative embodiment, a second refractory metal suicide layer is formed on the interior surface of the diode trench prior to deposition of the amorphous silicon. This second silicide layer, which is preferably titanium silicide, is used to decrease the resistance through the lightly doped end of the inventive diode.
Formation of the second refractory metal silicide layer is preferably accomplished by initially depositing a layer of sacrificial polysilicon on the interior surface of the diode trench. A blanket layer of titanium or some other refractory metal is then deposited over the polysilicon layer. Sintering is then used to form the two layers into titanium silicide.
The present invention also discloses other embodiments of novel vertical diodes having low series resistance. For example, in one embodiment the silicon wafer has an oxide layer with a hole etched therethrough to communicate with a silicon substrate. The silicon substrate is doped with a P-type dopant. The hole in the oxide layer is filled with a polysilicon plug that is heavily doped with an N-type dopant. The resulting silicon wafer is heated to a temperature sufficient to cause a portion of the dopants in the polysilicon plug to diffuse into the silicon substrate. As a result, a diode is formed having a junction located within the silicon substrate. If desired, a programmable resistor and metal contact can then be positioned on top of the polysilicon plug.
Finally, in yet another alternative embodiment, a vertical diode is formed by initially lightly doping a silicon substrate with a P-type dopant to form an active region. An oxide layer is then deposited over the silicon substrate. Holes are etched through the oxide layer down to the active region in the silicon substrate. The entire silicon wafer is then positioned within a reactor chamber where an epitaxial silicon layer is grown at the bottom of the holes against the active region. Once the epitaxial silicon layer is grown, the remaining portion of the holes are filled with a polysilicon plug that is heavily doped with an N-type dopant. The silicon wafer is then exposed to an elevated temperature that causes a portion of the dopants in the polysilicon plug to diffuse into a top portion of the epitaxial silicon layer. As a result, a diode is formed wherein the junction is positioned within the epitaxial silicon layer. As before, a programmable resistor and metal contact can then be positioned on top of the polysilicon plug.